Meteorological Fall Is Here: What Does That Actually Mean?

For some, September may feel like the unofficial start to fall, even though summer doesn’t officially end until later in the month. But in the weather world, fall is already well underway.

Meteorological fall officially began September 1. 🍂

Meteorologists and scientists define the seasons according to meteorological seasons and astronomical seasons.

Let’s break it down.

Meteorological vs. Astronomical Seasons

Astronomical seasons are probably the seasons we are all accustomed to growing up. They are based on Earth’s position in relation to the sun. In the Northern Hemisphere, astronomical fall begins with the autumnal equinox, which typically occurs around September 22 or 23.

Meteorological seasons are a little different.

Instead of following the position of Earth around the sun, meteorologists and climatologists divide the year into four three-month periods based on the annual temperature cycle and the calendar. Meteorological seasons help scientists track climate and weather trends. In layman's terms,it’s a lot easier to do the math.

Here’s how it works:

Meteorological winter: December, January and February
Meteorological spring: March, April and May
Meteorological summer: June, July and August
Meteorological fall: September, October and November

So, when I say meteorological fall has arrived, I’m talking about September 1 through November 30.

And there’s a very practical reason we do this.

Why Do Meteorologists Use Different Seasons?

The weather doesn't suddenly change because the calendar says it’s September 1.

We can have a heat wave in September. We can have an unusually chilly October. But when it comes to climate statistics, having consistent seasons is extremely important.

Imagine trying to compare fall temperatures from one year to another when the season begins on a slightly different date every year and can vary in length.

That would make the statistics much more complicated.

Meteorological seasons give scientists a consistent three-month period that can be compared from year to year.

For example, when NOAA tells us that a particular fall was warmer or wetter than average, we're generally looking at data from September, October and November.

That consistency makes it easier to calculate averages, identify trends and compare one season with another. NOAA's climate products routinely provide monthly and three-month seasonal summaries of temperature and precipitation, which is one reason these standardized periods are so useful.

Think of It Like Keeping Score

As a meteorologist, I often explain climate data by comparing today's weather with what is considered "normal."

Those normals are essentially our baseline.

NOAA's official U.S. Climate Normals are calculated using a uniform 30-year period and include monthly and seasonal statistics for temperature, precipitation and other climate variables.

So when we say something like:

"This was one of the warmest meteorological falls on record,"

we're not simply looking at one unusually warm afternoon.

We're looking at the collective temperature data from September, October and November and comparing it with historical records.

That gives us a much clearer picture of what the season actually looked like.

Why Not Just Use the Astronomical Seasons?

Astronomical seasons are incredibly important, too. They connect the seasons to the Earth's orbit and the changing angle of sunlight.

But the exact dates and lengths of astronomical seasons vary slightly from year to year. That's because Earth's orbit isn't perfectly synchronized to a 365-day calendar.

Meteorological seasons, on the other hand, are much more consistent.

That makes them particularly useful for record keeping, climate statistics, agriculture, energy planning and comparing weather patterns over time. NOAA notes that the more consistent length of meteorological seasons makes it easier to calculate and compare seasonal statistics.

In other words, meteorological seasons aren't about saying, "Fall feels like fall on September 1."

They're about creating a standardized way to measure and compare weather.

So What Can We Expect This Fall?

Now that we've officially entered meteorological fall, the weather pattern will gradually begin to evolve.

Days will continue to get shorter. The sun angle will gradually decrease. Temperatures will eventually trend downward, although that transition is anything but immediate.

And that's an important reminder:

Meteorological fall beginning doesn't mean summer weather suddenly disappears.

September can still be very warm—and in places like New York, it absolutely can feel like summer for a while.

In fact, some of my favorite fall days are the ones that give us a little bit of both seasons: cool mornings, comfortable afternoons and that first hint that a change is coming.

Eventually, the leaves will change, the nights will get longer and the chill in the air will become more noticeable.

But for now, we're in that beautiful transition period.

Summer isn't quite ready to let go.

And fall is patiently waiting its turn.

The Bottom Line

So the next time someone tells you, "Fall doesn't start until September 22," you can tell them they're talking about astronomical fall.

Meteorological fall is already underway.

Here's to crisp mornings, colorful leaves, sweater weather and, of course, whatever weather Mother Nature decides to throw our way. Happy Meteorological Fall! 🍁